IChO 2003 · Theory problem 16

Wavelengths of monochromatic radiation

Calculate the corresponding wavelength in nanometers (nm) for monochromatic radiation with the following numerical characteristics:

a) \(3000\ \text{Å}\) 150 nm ( ), 300 nm ( ), 600 nm ( ), 5000 nm ( )
b) \(5\times10^{14}\ \mathrm{Hz}\) 150 nm ( ), 300 nm ( ), 600 nm ( ), 5000 nm ( )
c) \(2000\ \mathrm{cm^{-1}}\) 150 nm ( ), 300 nm ( ), 600 nm ( ), 5000 nm ( )
d) \(2\times10^{6}\ \mathrm{GHz}\) 150 nm ( ), 300 nm ( ), 600 nm ( ), 5000 nm ( )

Official English text of the problem.

Answers

  • a) \(300\ \mathrm{nm}\)
  • b) \(600\ \mathrm{nm}\)
  • c) \(5000\ \mathrm{nm}\)
  • d) \(150\ \mathrm{nm}\)

Solution

Use the speed of light in vacuum, \(c\approx3.00\times10^{8}\ \mathrm{m\,s^{-1}}\), and the relationships \[ \lambda=\frac{c}{\nu}, \qquad \lambda=\frac{1}{\tilde{\nu}}, \] where \(\nu\) is frequency and \(\tilde{\nu}\) is wavenumber.

a) Since \(1\ \text{Å}=10^{-10}\ \mathrm{m}=0.1\ \mathrm{nm}\), \[ \lambda=3000\ \text{Å}\times\frac{0.1\ \mathrm{nm}}{1\ \text{Å}} =\boxed{300\ \mathrm{nm}}. \]

b) Since \(\mathrm{Hz}=\mathrm{s^{-1}}\), \[ \lambda=\frac{3.00\times10^{8}\ \mathrm{m\,s^{-1}}} {5\times10^{14}\ \mathrm{s^{-1}}} =6\times10^{-7}\ \mathrm{m} =\boxed{600\ \mathrm{nm}}. \]

c) The quantity in \(\mathrm{cm^{-1}}\) is the wavenumber. Taking its reciprocal gives the wavelength in centimetres: \[ \lambda=\frac{1}{2000\ \mathrm{cm^{-1}}} =5\times10^{-4}\ \mathrm{cm}. \] Using \(1\ \mathrm{cm}=10^{7}\ \mathrm{nm}\), \[ \lambda=5\times10^{-4}\ \mathrm{cm}\times \frac{10^{7}\ \mathrm{nm}}{1\ \mathrm{cm}} =\boxed{5000\ \mathrm{nm}}. \]

d) First convert gigahertz to hertz: \[ \nu=2\times10^{6}\ \mathrm{GHz}\times \frac{10^{9}\ \mathrm{Hz}}{1\ \mathrm{GHz}} =2\times10^{15}\ \mathrm{Hz}. \] Therefore, \[ \lambda=\frac{3.00\times10^{8}\ \mathrm{m\,s^{-1}}} {2\times10^{15}\ \mathrm{s^{-1}}} =1.5\times10^{-7}\ \mathrm{m} =\boxed{150\ \mathrm{nm}}. \]

The answers were checked against the official answers published by the organisers.